The present invention relates to cationic electrodepositable compositions and to the use of these compositions in the method of cationic electrodeposition.
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because by comparison with nonelectrophoretic coating means, electrodeposition offers higher plant utilization, outstanding corrosion protection and low environmental contamination. Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
In preparing the paint composition which is used in the cationic electrodeposition process, a resinous binder which contains basic groups, such as basic nitrogen groups, is neutralized with an acid. The resultant cationic resin is dispersed in water and combined with pigment and other additives normally used in the cationic electrodeposition process to form a paint. Typically, the neutralizing acids are organic acids such as acetic acid and lactic acid because these acids provide for the most uniform and stable dispersions. Although the use of inorganic acids such as nitric, sulfuric and phosphoric acid are known as neutralizing agents in the cationic electrodeposition process, these acids do not normally provide satisfactory dispersion for the cationic electrodeposition process as practiced industrially.
In accordance with the present invention, it has been found that sulfamic acid can be used as a neutralizing agent for the cationic coating composition. The resultant dispersion has a small uniform particle size giving it enhanced stability resulting in less settling and a cleaner product. Also, cationic electrodeposition coating compositions can be made to deposit as relatively thick coatings with improved throwpower. In addition and somewhat surprisingly, the sulfamic acid passivates the anode against the corrosivity of anolytes normally associated with cationic electrodeposition compositions which are neutralized with typical organic acids such as lactic acid and acetic acid.